1. Field of the Invention
This invention relates to the sensing of physical parameter, such as pressure or temperature, using in-fiber sensors, e.g., short period fiber Bragg grating, long period fiber grating or surface corrugated long period fiber grating. Since signal sensing and signal transfer can both occur in the same fiber, it is possible to derive miniature pressure or temperature sensing systems or transducers for use in medical diagnoses in the areas of intradiscal, intracranial, intramuscular, intra-articular, ventricular pressure or temperature monitoring with minimum invasiveness. For example, for intra-articular pressure measurement, the small size of the needle means that the pressure of small joint cavities like the temporo-mandibular joint, the facet joints of the vertebral column, the carpal joints of the wrist and the tarsal joints of the midfoot region can also be assessed by means of this device. With respect to the technology background of the invention, please refer to the following references:    [1] Pollintine P, Przybyla A S, Dolan P, Adams M A. Neural arch load-bearing in old and degenerated spines. J Biomech 2004;37:197–204.;    [2] Sato K, Kikuchi S, Yonezawa T. In Vivo intradiscal pressure measurement in healthy individuals and in patients with ongoing back problems. Spine 1999;24:2468–74;    [3] Wilke H J, Neef P, Caimi M, Hoogland T, Claes L E. New in-vivo measurements of pressures in the intervertebral disc in daily life. Spine 1999;24:755–62;    [4] McNally D S, Shackleford I M, Goodship A E, Mulholland R C. In vivo stress measurement can predict pain on discography. Spine 1996;21(22):2580–7;    [5] Kane; James, “Optical pressure sensor for measuring blood pressure”, U.S. Pat. No. 4,691,708;    [6] Wallace L. Knute, Wilber H. Bailey, “Fiber-optic transducer apparatus”, U.S. Pat. No. 5,107,847;    [7] Alderson; Richard, “Fiber optic coupled pressure transducer using single fiber and method of fabrication”, U.S. Pat. No. 4,711,246;    [8] U.S. Pat. No. 4,924,870;    [9] U.S. Pat. No. 5,275,053;    [10] U.S. Pat. No. 5,385,053; and    [11] U.S. Pat. No. 5,422,478.
2. Description of the Prior Art
Pressure measurement is important in engineering, medical diagnosis and research and development in many fields. Technology used in conventional pressure measurement may be broadly differentiated into mechanical, electrical and fiber-optic categories. A diaphragm that can deform under the application of pressure is normally employed as the primary transducer for pressure. The deformation of the diaphragm is converted into the movement of a dial pointer through suitable mechanisms in the mechanical pressure gage. The use of purely mechanical components makes this type of gages very bulky. In the electrical category, resistive strain gages are normally employed to convert the diaphragm deformation into electrical signals. Although this type of pressure transducer can be made much smaller than the mechanical ones, the size of the strain gages and the need to lead out a number of wires for electrical excitation and signal measurement make it difficult to reduce their size to below the millimeter level. Typical small-sized pressure probe of this category used for in-vivo medical measurement has a diameter from 1.5 to 3 mm, please refer to references [1] to [4]. At these sizes, the implantation of the pressure transducer to make in-vivo measurement is a rather invasive procedure and could mean much discomfort to the subject concerned. This type of transducer is also susceptible to electromagnetic interference and measurement accuracy may be affected if there are other medical instruments nearby.
Referring to references [5], [6] and [7], the typical fiber-optic pressure transducer system comprises two sets of optical fibers. One set of fibers transmits a light beam to shine on the deformable diaphragm and the other set of fibers returns a modulated light beam reflected from the diaphragm. It is also possible to use a single optical fiber for the two-way light traffic, please refer to reference [7]. Pressure variation will deform the diaphragm, thereby varying the proximity of the diaphragm to the fiber ends, thus modulating the intensity of the reflected light. By measuring the reflected light intensity using a photo-sensor, the pressure can be deduced. Good alignment of the fibers and the reflecting surface of the diaphragm is needed and high precision manufacturing process make this kind of sensor expensive to produce. This type of transducers have not been entirely satisfactory as the intensity of light transmitted in an optical fiber can be reduced by bending of any part of the fiber, movement of the pressure probe and a faulty connector. Moreover, temperature fluctuation may also affect the accuracy of the measurement. In fact, there are a number of inventions made to combat these problems, please refer to references [8], [9], [10] and [11], but this often means packing some more optical fibers into the transducer for reference purposes.